Good luck! I love the name Isla. My top three combos personally are IslaCharlotte, IslaJane and IslaHarper. But I really want to use a literary middle name for my daughter, so those three are high on the list.

IslaJune maybe? Pretty sure that doesn't fit the Scottish. I actually like IslaJean. IslaKatherine would give you more play for nicknames. Or you could pick two (IslaRubyJean) then she'd have an option to drop one if she didn't totally love it